Measuring skeletal muscle oxidative metabolism has been important in understanding muscle function in health and disease. For example, it is important to be able to measure muscle oxidative metabolism in spinal cord accident victims both before and after therapy, which may include the administration of one or more drugs that are under evaluation.
Noninvasive methodologies have enhanced the study of mitochondrial function, particularly in human participants. The primary noninvasive method of measuring mitochondrial function has been magnetic resonance spectroscopy (MRS) and more particularly the use of the kinetics of phosphocreatine (PCr) resynthesis after exercise as a direct assessment of mitochondrial capacity. Although MRS can provide an accurate indication of mitochondrial capacity, MRS is a costly technique that requires large, expensive equipment and a high level of technical expertise.
Near-infrared spectroscopy (NIRS) also provides a noninvasive measure of muscle oxygenation. Commercially available NIRS devices typically provide information about the relative changes in oxygenated hemoglobin/myoglobin (O2Hb), deoxygenated hemoglobin/myoglobin (HHb), and total hemoglobin or blood volume (tHb). In comparison to MRS equipment, NIRS devices are much smaller, less expensive, portable, and easier to use, making them more practically useful for a clinical setting.
NIRS measurement of skeletal muscle oxygen consumption has been conducted using both venous and arterial occlusions. In the typical procedure, blood flow through the veins and arteries is halted by the application of a cuff, and measurements are then taken with the NIRS device to quantify the oxygen that is being consumed by the muscle. Although it is typically assumed that the cuff stops all blood flow, in reality blood still flows into the smaller blood vessels, such as the capillaries, due to the pressure differential between the blood within the arteries and the smaller vessels. Such blood flow is associated with a change in blood volume that can skew the other measurements taken by the NIRS device. In order for the arterial occlusion method of measuring skeletal muscle oxygen consumption to be accurate, there should be little or no change in blood volume, tHb.
From the above discussion, it can be appreciated that it would be desirable to have a system and method for measuring skeletal muscle oxygen consumption using NIRS that compensates for changes in blood volume and, therefore, provides more accurate measurements.